1. Field of the Invention
The present invention relates to a sheet processing apparatus that is detachably equipped on the main body of an image forming apparatus such as a copying machine or a printer, and to an image forming apparatus equipped with such a sheet processing apparatus. Particularly, the present invention relates to a sheet processing apparatus that can discharge sheets reliably and an image forming apparatus equipped with such a sheet processing apparatus.
2. Related Background Art
Recently, sheet processing apparatuses such as sorters for sorting sheets, on which images have been formed, have been developed as optional apparatuses for image forming apparatuses such as electrophotographic copying machines or laser printers. Such kinds of sheet processing apparatus is designed to perform at least one processing such as sorting, stapling or jogging etc. on sheets.
A sheet processing apparatus having a stapler for stapling sheets is designed in such a way that the stapling operation is performed after the sheets conveyed into the body of the sheet processing apparatus have been made to pass through a conveying path provided in the interior of the body and then stacked on a post-processing tray.
The sheet processing apparatus for stapling a stack of sheets is adapted to place sheets in a stack on the post-processing tray and moving a stapler serving as stapling means to perform stapling at one position or multiple positions (typically, at two positions). During the stapling operation, it is not possible to place the sheets for the next job on the post-processing tray. Consequently, it is necessary to set intervals between sets of sheets on a job by job basis for stapling operations.
However, the intervals set between sheets invites a decrease in productivity. In other words the number of sheets processed per unit time is decreased. A sheet processing apparatus as shown in FIG. 31 has been proposed as a sheet processing apparatus that will avoid such a decrease in productivity (see, for example, Japanese Patent Application Laid-Open No. H9-48545).
The conventional sheet processing apparatus 10 shown in FIG. 31 has a buffer roller path 14 provided in a conveying path 12 in the midway of the sheet conveying path to a post-processing tray 11. In the buffer roller path 14, sheets are wrapped around a rotating buffer roller 13 so as to be kept in a waiting state before conveyed to the post-processing tray 11.
With the above-described structure, in the conventional sheet processing apparatus 10, sheets conveyed from a sheet discharge roller pair 17 provided in the main body 16 of an image forming apparatus 15 are stored in the buffer roller path 14, so that a set of sheets stored on the buffer roller 13 are conveyed to the post-processing tray 11 after the processing, such as stapling for example, of the preceding set of sheets on the post-processing tray 11 has been completed and the preceding set of sheets has been discharged from the post-processing tray 11 by a rotating upper roller 18a of an oscillation roller pair 18 that holds the sheets in cooperation with a lower roller 18b. Thus, the intervals between sheets are not extended during the stapling operation, and a decrease in productivity can be avoided.
However, with the provision of the buffer roller path 14, this conventional sheet processing apparatus 10 suffers from the problem that it is necessary to arrange an installation space for the buffer roller 13 and the buffer roller path 14 for suspending the conveyance of the succeeding sheets to the post-processing tray 11 so as to keep them in a waiting state during the stapling operation, and so the size and the cost of the sheet processing apparatus are increased.
In addition, in the prior art sheet processing apparatus 10, since sheets are discharged by means of the oscillation roller pair 18, the sheet conveyance rate can vary due to a variation in the friction between the upper roller 18a and the lower roller 18b or a variation in the rotation speed of them, so that a displacement between the upper portion of the sheets and the lower portion of the sheets can occur. Therefore, sheet discharge operation is not stable and there is a variation in the time required for discharging sheets.
Furthermore, even the conventional sheet processing apparatus 10 that is adapted to place the sheet stored in the buffer roller path onto the post-processing tray 11 after the sheets on the post-processing tray 11 have been discharged cannot meet recent requirements for an increase in the processing speed, and an apparatus with a reduced processing time has been demanded.